Canned tins were produced in Iran for the first time in 1309 by a person named Derakhshan. He, who was one of the immigrants living in the Soviet Union, returned to Iran after the October Revolution, and because he was educated at the Moscow Cooking School, using his experiences, he started the first canning workshop in Iran. By using the empty gasoline pits of the oil company and flattening them by hand, he passed the obtained sheet through the molten tin and made cans from them. In 1336, ready-made lacquer sheets and cans were imported from abroad to Iran, and in 1344, today’s automatic lacquering was installed and operated for the first time in Iran. Although the total lifespan of canning in Iran does not exceed 60 years, it can be considered when compared to the lifespan of this industry in the world (about 180 years).
With the first methods, a good and skilled worker produced four cans per day, while today the production speed of advanced canning lines even reaches more than a thousand cans per minute. During the years after the emergence of the canning industry, many changes have been made in the machines and knowledge of this technology, and we always see the progress and hear new news in this field. Increasing the production speed to more than one thousand cans per minute, increasing the strength of the cans, creating various shapes and sizes, using printed decorations on the cans, and inventing various doors known as Easy Open, in addition to Weight reduction of cans are the main indicators of canning progress.
Metal can production methods
The production of cans made of coated tin sheet, contrary to the public opinion that it is easy, requires knowing many new skills and technologies of the day. The accuracy and dimensions of parts in production equipment and machines are subject to micron tolerances, which places this industry in the group of precision industries. The most important types of cans are food or non-food metal cans in three pieces or two pieces.
A- 3-piece can
1. Can with welded body: after being rolled, the can body becomes a cylinder with ERW (Electrical Resistance Welding) method.
2. Can with laser welding body
3. Can with solder seam body: This method has been abandoned due to contamination caused by soldering alloy.
4. There are other methods for sewing the seam of the can body, which are not common in our country.
B- 2-piece can
1. Double can (DI: Drawn & Wall Ironed can: long double cans that are formed by the operation and ironing of the sheet in a stretch form, and their bottom and body are of the same material.
2. Two-piece stretch cans: Two-piece cans are produced with a stretch operation with a lower stretch ratio and material flow into the mold, and the body and bottom of the can are of the same material.
Three-piece can production steps
1. Varnishing of coated tin sheet in the varnishing line
2. Cutting the can body parts (Body Blank) in the Doublex Slitter machine
3. Can body cylinder production in the body making machine
4. Covering the weld of the body from the inside with powder or varnish in an electrostatic way to protect this part from corrosion
5. Baking and sticking the powder (or varnish outside the seam) to the body cylinder in a special oven (Seam Coating Curing Oven)
6. Performing operations: dividing the body (Parting), beading, changing the shape of the cross-section of the can, turning the edge of the body (Flanging), sewing the bottom to the body (Seaming) which is done in one or more steps and in one or more machines with Pay attention to the need and type of equipment used at the end of the production operation.
Two-piece can production steps
1. Varnishing of coated tin sheet in the varnishing line
2. Cutting body sheets in zigzag shape or simple or without cutting in sheet feeding method
3. Feeding the coated tin sheet into the press and producing a two-piece stretch can with a turned edge in a special press where drawing and redrawing, trimming and turning the edges are done at the same time. Most canning lines in Iran have been equipped with new and modern machines during the last decade. The tendency to use thinner coated tin sheets in order to save on the consumption of raw materials is one of the main characteristics of these lines.
Reducing the thickness of sheets, along with increasing the speed of production, requires as much compliance as possible with the different qualitative ranges required in the production of raw materials, especially coated tin sheets. It should be mentioned that the design printed on the two-piece stretchable cans is designed with a special software. In these types of cans, the design printed on the sheet finds its original shape after being stretched. The Spanish company Logitech is one of the centers that provides this service, and many Iranian companies order their designs there.
The points of interest and the problems and limitations related to coated tin sheet in the canning industry are briefly as follows:
1. The specifications of the used sheet recommended by the machine manufacturer in can body making machines, especially high speed machines, should be carefully observed.
2. It is mandatory to observe the rolling direction in the can body production lines with the resistance welding method.
3. Non-observance of dimensional tolerances in a bundle of sheets will cause defects in varnishing operation and creation of a precise strip without varnish (margin) and body scissors cutting.
4. The operation of turning the edges for sheets with a hardness greater than a certain limit is done with the spinning method (Spin Flanging), while this operation is done for sheets with temper and low hardness with the method of irons (Die Flanging), the cost The parts and equipment of the rotary method are much more than the second method. The spring back behavior of the sheet (Spring Back) in the same conditions in terms of parts and production machines, changes with the change in the hardness of the sheet, and the sewing factors of the can reveal the changes as the most basic quality factor of the product. The profile of the parts used to create beads in the body of the can is fixed, and the changes in sheet hardness have a significant effect on the depth of these beads, and as a result, the moment of inertia obtained and the resistance against the internal pressures of the filled can.
5. Most of the machines of the can industry in Iran are fed with tin-coated sheets, and due to the lack of lacquer and roll printing facilities in the country, this type of product is not used directly.
6. Tin: Research shows that an increase in tin requires an increase in welding current. The changes outside the standard limits are from the quality of body seam welding and its resistance to various pressures caused by food processing. Although tin coating is mainly used on the base sheet to increase the resistance of the sheet against corrosion, the ability to weld and solder and varnish and to create a pleasant appearance, the experiences obtained show that the presence of tin also reduces the wear of production parts and shape It increases and improves the flexibility of the sheet. Although the increase of tin is the main factor in the increase in the finished price of the sheet and has an inverse relationship with the welding flow, it has a useful and direct role in the stretching cans and the ironing process.
One of the phenomena that can be mentioned about tin sheets is the sticking of tin particles in the form of cold welding to the production parts. Over time, these particles stick to the parts that are responsible for shaping the sheet, which need to be removed. Parts such as seaming chucks, seaming rollers, and body builder rollers are subject to this problem.
7. Passivation: This work is done to increase the resistance of the surface of the sheet against oxidation and improve the quality of varnishing and printing. Two methods are common for deactivation: in method 311, sodium dichromate solution is placed on the sheet of the cathodic face. Most manufacturers use this method. Method 300, which is easily done by immersing the sheet in sodium chromate solution, is no longer considered due to environmental pollution in countries where pollution of the living environment, water and soil is of great importance.
8. Oiling: By referring to the health instructions of the industries that deal with coated tin sheets, breathing and continuous contact with metal particles and gases resulting from the various processes of producing and shaping such sheets with welding, melting, cutting, stoning and Machining, due to the presence of carbon, chromium, iron, manganese, nickel, silicon and tin particles, requires compliance with safety tips and measures to cause less complications. Among the things that are effective in reducing the dust particles caused by the operation on the coated tin sheets, is the use of permitted and suitable oils for the food industry, which is used at this stage, this method also prevents the absorption of moisture from the environment. .
The amount of oil and its uniform distribution and life are very important in the quality of varnishing and sheet coating operations. It is better to control and examine this oil with conventional methods before entering the coating process. Some of these methods are as follows:
1. Water-break test: It is the easiest method to ensure the presence of oil on the sheet.
2. Ellipsometer test: It is the most common method to determine the thickness of oil, which is done with the help of a device with the same name based on optical principles.
3. Soxhlet method test: In the past, the main method of determining the thickness of oil on a coated tin plate was considered, and now it has been replaced by easier and faster methods. This method is described in the ASTM A623 standard and it is based on removing the oil from a certain surface of the sheet with the help of a boiling solvent such as chloroform and measuring the remaining oil after the evaporation of this solvent.
9. Packaging of coated tin sheet: Improper packaging and the ability to penetrate the humidity of the environment until the time of use, along with the strong permeability caused by the surface tension property between the sheets, causes irreparable damage to this industrial production.